Dense wavelength division multiplexing (DWDM) in optical networking systems offers a very efficient method of exploiting the available bandwidth in the low attenuation band of the optical fiber, which includes C-band from 1520 nm to 1565 nm, L-band from 1565 nm to 1610 nm and S-band from 1450 nm to 1520 nm, respectively. In this technology, the enormous available bandwidth is chopped into a number of parallel wavelength channels, where each channel carries data up to a maximum rate compatible with electronic interfaces. Different protocols and framing may be used on different channels. This is very similar to frequency division multiplexing (FDM) used for radio and TV transmissions.
While DWDM technology has progressed in the past few years, it can progress further. The progression of DWDM technology has increased the number of feasible channels in the total band. Early WDM systems used only 4 to 16 channels, while current and next generation systems are targeting more than 100 channels. For the transmitters of a DWDM system, there are a number of different laser sources with different wavelengths. Each data channel is modulated on one of the wavelength channels and all the wavelength channels are then multiplexed and transmitted via the same optical fiber. At the receiving end, each channel must be demultiplexed from the set of wavelength channels. An optical receiver will then demodulate data from each channel. The capacity of a DWDM system increases as many wavelength channels are provided. It is therefore desirable to thereby not only increase the number of channels but also to increase the total wavelength bandwidth.
Current laser sources used in DWDM systems are exclusively of the single-wavelength variety. Distributed Feed-Back (DFB) lasers, Fabry-Perot lasers and ring lasers are some of the main technologies for such laser sources. In these technologies, each wavelength supported in the system has a dedicated laser and its ancillary electronics. Most recently, designs are making use of tunable wavelength lasers, which have a broader spectral range and can operate at any point within that range. The primary drawback of these laser sources is the sheer number required to satisfy high channel count systems proposed for the future optical network. As a result, a simple and compact simultaneous multi-wavelength laser system (MWLS) with a high number of channels that are matched to an International Telecommunication Union (ITU) grid, low noise, large operation wavelength range, wide intensity uniformity and excellent stability is highly desirable in DWDM optical networking systems. But to date, there has not been any simple and compact MWLS with a large operation wavelength range which covers C-band, L-band, and S-band wavelengths.
In the past ten years, the use of erbium-doped fiber (EDF), bulk or quantum-well (QW) semiconductor waveguide as gain materials for simultaneous CW MWLS has been very hot research topics. Prior efforts have demonstrated MWLS oscillations with equal frequency spacing by using active overlapping linear cavities, a high birefringence fiber loop mirror, intracavity polarization hole burning or distributed Bragg grating, an elliptical fiber, intracavity tunable cascaded long-period fiber gratings or a sampled chirp fiber Bragg grating, and a self-seeded Fabry-Perot laser diode, spatial mode beating within the multimode fiber section and multi-cavity oscillation.
Regardless of the above efforts, through the use of bulk or QW semiconductor gain materials, some research groups have successfully generated simultaneously multi-wavelength laser outputs. Because of the relatively large homogeneous gain broadening of EDFs and bulk or QW semiconductor waveguides at room temperature, simultaneous multiwavelength lasing in these gain materials is very sensitive to variations in cavity losses. Homogeneous broadening implies that the gain provided by the EDF and bulk or QW at one wavelength uniquely determines the gain at all other wavelengths.
Any change in the wavelength dependence of cavity losses will, at some of the lasing wavelengths, typically break the requirement that the gain equal the cavity losses.
Thus, lasing stops at those wavelengths. Moreover, since the gain spectrum depends on the operating level of the gain, wavelength-independent loss variations also normally break the required gain-loss balance, with the same result. Currently reported tunable simultaneous multi-wavelength lasers therefore require careful balancing of cavity losses at each wavelength, particularly if large numbers of wavelengths are to be generated. Another problem is the covering wavelength range of the current reported MWLS is not wide enough for some special applications. It would therefore be very desirable to develop a MWLS with a stable output which covers a large wavelength range.
Optical networks, which would use the MWLS, can be viewed as a three-level hierarchy consisting of backbone networks, metro networks, and access networks. Future backbone networks provide enormous bandwidth and high data rate and could be based on DWDM and optical time-domain-multiplexed (OTDM) links. Access networks transport data to or from individual users. Metro networks play an important role by interconnecting both of them so that direct optical connections can be established. All-optical networking potentially allows high-speed optical communications to become more cost effective by the use of low-cost transparent light paths, which do not need any optical-to-electrical (O/E) and electrical-to-optical (E/O) conversions. In all optical networking systems, optical wavelength conversion is anticipated as being essential for increasing the efficiency and throughput of high-speed DWDM optical networks by enabling rapid resolution of output-port contention, lightpath failure, as well as wavelength reuse. Besides its function of removing the wavelength continuity constraint, wavelength converters may be used to promote interoperability and distribution of network control and management functions across sub-networks. A flexible and independent wavelength converter with input and output wavelength tunability will enable DWDM networks to have improved operation flexibility through a simplified wavelength-routing algorithm, reduced blocking probability, and other enhanced performance metrics.
Wavelength multicasting is another technology which would benefit from a workable MWLS as described above. Wavelength multicasting involves copying an input signal onto many different output signals such that the data can traverse parallel paths to reach the destination. Many bandwidth-intensive applications such as video distribution and teleconferencing require reliable high-speed multicasting. In a wavelength-routed DWDM network, wavelength multicasting would require an input channel to be simultaneously replicated onto multiple selected output wavelengths and would be a laudable function to achieve. Existing technologies for all-optical wavelength multicasting are in early research and development stages. These existing technologies include: 1) cross-phase modulation (XPM) and cross-gain modulation (XGM) in QW semiconductor optical amplifiers (SOAs); 2) cross-absorption modulation (XAM) in an electroabsorption modulator (EAM); and 3) XPM and four-wave mixing (FWM) in highly nonlinear fiber (HNLF). The limitations and drawbacks of the above mentioned existing wavelength multicasting systems are: the operation speed is limited by the carrier dynamics when using XGM in QW SOAs; inherent asymmetry of the wavelength conversion efficiency in their detuning characteristics when using FWM in QW SOAs; modulation format dependence exists for XGM, XAM and XPM techniques; poor noise performance and low extinction ratio, small operation wavelength range and noticeable power penalty.
It is therefore an object of the invention to provide methods and devices that overcome or at least mitigate the drawbacks and shortcomings of the prior art.